Forces

A push or a pull.
◦ When one object pushes or pulls another
object, the first object is exerting a force on
the second object.
◦ Like velocity and acceleration, force is also
described by the strength and direction in
which it acts.
◦ The strength of a force is measured in an SI
unit called Newton (N). (named after Isaac
Newton)
What is a Force?

In any situation the overall force
acting on an object is the net
force.

The net force determines
whether an object moves and
also in which direction it moves.
◦ Force in the same direction add
forces.
◦ Force in opposite direction is like
adding positive and negative
numbers. They cancel each other
out.
◦ Size of the arrows indicates the
strength and direction of the force.
Combining Forces

Unbalanced force acting on the object
results in a net force which will change
the objects motion.

Balanced forces equal forces acting on
one object in opposite directions.
Balanced forces acting on an object will
NOT change the objects motion.
Unbalanced vs. Balanced
Unbalanced vs. Balanced
Friction

The force that two surfaces exert on each
other when they rub against each other.
◦ Acts in the direction opposite to the objects
motion. Friction opposes motion.
◦ Eventually friction will cause an object to come
to a stop.
◦ Without friction the object would continue to
move at a constant speed until acted upon by
another force.
What is Friction

The amount of friction depends on two
factors
1. Surface Type

Rougher surfaces have greater friction

Less massive objects exert less force.
2. The amount of force pushing surfaces
together.

Changing the amount of the surface
(surface area) that touches does not
change the amount of friction.
Determining the Amount of Friction
Salt, sand or gravel on icy surfaces
 Tires on a car
 Boots or shoes with traction
 Other examples?

When is Friction Helpful?

Static friction-the friction that acts on objects that are
not moving
◦ Ex: Moving a desk. It takes more force to get it moving
(overcome static friction)

Sliding friction-when solid surfaces slide over each
other
◦ Ex: Sliding a box across the floor, the breaks on your bike
tire.

Rolling friction-when an object rolls over a surface

Fluid friction-when an object moves through a fluid
◦ Ex: Skateboards, bikes. Anything with wheels!
◦ Ex: Biking, surfing, swimming.
Types of Friction

Reduce
Lubricants

◦ Ex: motor oil, wax,
grease.

◦ Sand on icy roads,
batting gloves.
Switching to rolling
◦ Ball bearings.

Increase
Make surfaces
rougher.
Smoother surface
◦ Plastic as opposed to
wood.

Increase amount of
force pushing
surfaces together.
◦ Add weights.
Ways to Reduce/ Increase Friction

Physics of The Olympics
◦ Competition Suits
◦ Curling
Friction Video Clips
Gravity

The force that pulls objects toward each
other.
What is Gravity?

“All objects in the universe attract each
other through gravitational force . The
size of the force depends on the masses
of the objects and the distance between
them.”
◦ Sir Isaac Newton (1642-1727)
The Law of Universal Gravitation

Mass- The more mass
an object has the
greater it’s
gravitational pull.
◦ Ex: The sun has a large
mass which keeps the
planets in orbit

Distance- The farter
apart two objects are,
the lower the
gravitational force
between them.
Factors that Affect Gravity

When the only force
acting on a falling
object is gravity, the
object is said to be
in free fall.

The acceleration of
any object due to
gravity is 9.8 m/s/s
(ignoring air resistance).
Free Fall

Mass is the amount of matter in an object.
◦ Measured in grams or kilograms

Weight is the measure of the gravitational
force exerted on an object.
◦ Measured in Newtons

Why do we weight less on the moon?
◦ Because the moon has less mass than the earth.
Mass vs. Weight
Moon Weight

Weight = mass X acceleration due to gravity
(9.8 m/s/s)

How much does a 50 kg person weigh on
earth?
◦
◦
◦
◦
Weight = mass X 9.8 m/s/s
Weight = 50 kg X 9.8 m/s/s
Weight = 490 Newtons
Where force (weight) is measured in Newtons,
acceleration is measured in m/s/s and mass is
measured in kg.
Try it out…
1.
Convert lbs. to kg
(Weight in pounds X 0.4536)
2.
Convert kg to Newtons
(Weight in kg X 9.8)
Ex. 190 lbs X 0.4536 = 86.18 kg
86.18 kg X 9.8 = 844.60 N
To get weight on moon just divide by 6.
How many Newtons do you
weigh?

An object that is thrown is called a
projectile.
Projectile Motion
Which Will Hit The Ground First?

A type of fluid friction.
◦ The greater the surface area the greater the air
resistance.
◦ The greater the velocity the greater the air
resistance.
Air Resistance

As velocity increases air resistance
increases. When the force of the air
resistance equals the force of the gravity
on an object, the object will no longer
accelerate. It will continue to fall at a
constant rate.

So, terminal velocity is the point at which
a falling object stops accelerating and
begins to fall at a constant speed.
Terminal Velocity

Exploring Gravity